An electrochromic system with polymeric solid electrolyte generally comprises two electrodes between which is sandwiched a thin layer of an ion-conducting macromolecular material (polymeric solid electrolyte), especially a conductor of protons or of alkali metal ions, at least one of the said electrodes containing a material, called an electrochromic material, which is placed facing the layer of ion-conducting macromolecular material and in which the insertion or deinsertion of ions, especially protons or alkali metal ions, results in a change in the light absorption and/or reflection spectrum of the said material, these electrodes, on the one hand, being in direct contact with the layer of polymeric solid electrolyte to which they adhere strongly and, on the other hand, each comprising a currentconducting deposit which is in contact with one of the facing sides of two support plates situated on both sides of the said electrodes and at least one of which and the associated conducting deposit are transparent. In particular, each electrode comprises a transparent current-conducting deposit, for example a deposit based on mixed indium tin oxide, which is in contact with one of the facing sides of two plates made of an inorganic or organic transparent material such as glass or transparent plastic, which are situated on both sides of the said electrodes and are coupled to form a window-type structure.
When a suitable potential difference, generally lower than 5 volts, is established between the conducting deposits of the electrodes, the appearance of a permanent colouring of the electrochromic system is observed, and this system can again become colourless on the application of an electrical voltage, generally of opposite sign to that which produced the colouring.
An electrochromic system of the abovementioned type can be employed as a window with variable light transmission for domestic use or for motor vehicles or as a mirror or rear-view mirror with variable light reflection.
It is advantageous to be able to heat such a glazed surface uniformly, to permit demisting, defrosting and/or a faster response of the colour change to the electrical voltage which is applied
The use of an external source of heat for heating an electrochromic system of the abovementioned type does not allow the required result to be obtained because operating in this way results in the appearance of a temperature gradient inside the electrochromic system, due to the poor diffusion of heat in the multilayer structure forming the said system, and this is reflected in a nonhomogeneous operation of this system.
It has already been proposed, as described in reference GB-A-2,065,027, to perform the heating of a polymeric composition forming a thin layer and containing an ion-conducting macromolecular material consisting of a polyether coupled with an ionisable salt by relying on a heating technique using dielectric losses, which consists in subjecting the said composition to the action of electromagnetic waves of very high frequencies, namely frequencies of the order of 10.sup.6 to 10.sup.8 hertz.
Such a heating technique using dielectric losses is not suitable for heating an electrochromic system such as referred to above, which comprises a thin layer of an ion-conducting macromolecular material sandwiched between two structures with high electronic conduction, namely the electrodes of said system, because, apart from the difficulties linked with its implementation and the disadvantages which it entails for the environment owing to the use of electrical signals of very high frequency, this technique does not lend itself well to heating multilayer structures comprising a number of layers with high electronic conduction which are close to each other.